1. Field of the Invention
The present invention relates to hydraulic systems that have a single acting hydraulic cylinder, and more particularly to an arrangement of valves for controlling operation of such a hydraulic cylinder.
2. Description of the Related Art
A variety of agricultural implements are available to be pulled by a tractor for working earth in a field in which crops will be or have been planted. The implement is connected to a standard three-point hitch on the rear of the tractor and the hitch can be hydraulically operated to raise and lower the implement.
In order to raise or lower the tractor hitch, the operator manipulates a human interface in a manner that indicates the direction and speed at which the hitch is desired to be moved. The human interface sends an electrical signal to a controller that then applies an electric current to a solenoid operated directional control valve. That directional control valve either sends pressurized fluid to one or more single-acting cylinders to raise the hitch or drains fluid from the hydraulic cylinders to lower the hitch.
The directional control valve often is a three-position spool valve in which a closed, or center, position is between the two positions that provide the raised and lower states of the valve. It is desirable that a closed position be in between the two active positions so that an abrupt reversal of hydraulic operation does not occur, as would happen if the raise position was immediately adjacent the lower position. The directional control valve often is a spool valve with a pair solenoids acting on opposite ends of the spool. Activating one of the solenoids moves the control valve in one direction to raise the hitch and energizing the other solenoid moves the spool in the opposite direction to lower the hitch. Springs at those opposite ends center the spool into the closed position when both solenoids are de-energized.
Such dual solenoid, spool valves are commonly used in hydraulic systems for a wide variety of machines. Although spool valves are acceptable in a many applications, there is a certain amount of leakage through the valve in the closed position. That leakage is of little concern in most applications because the valve is operated so frequently that it does not remain in the closed position long enough for a significant amount of total leakage to occur that adversely affects operation of the machine.
That, however, is not necessarily the case with respect to a spool-type directional control valve used to position the hitch of an agricultural tractor. When the tractor commences to work an agricultural field, the hitch positions the implement at a desired depth in the soil. The tractor may then be operated for a significant amount of time in that position of the implement. For example, if the implement is a plow, the hitch may remain in the same position while a field of many acres is being plowed. During that length of time, the total leakage occurring in the spool-type valve can be great enough to produce a significant deviation in the position of the implement. The vehicle can be parked for a significant period of time, during which the hitch is in an extended condition with an implement raised. At such times, the hydraulic system is not active to provide pressurized fluid to maintain the hitch and implement in the raised position. As a result, a load holding technique must be provided to prevent the hitch from lowering under the hydraulic load.
Therefore, there is a desire to provide a hydraulic system has minimal leakage when the hitch remains in a single working position for a prolonged period of time. It further is desirable to control the directional spool valve with a single solenoid to reduce complexity of that device.